Hit support with it. I had the same thing setting up an account that was NOT a gmail account. Here are two sources for root certificates in case that is the problem (which it often can be) It did not help my issue this time however

After problems with The bat! v6.4.0 ( same as: http://www.ritlabs.com/en/forums/forum4/topic11531/ ), suddenly it doesn?t works, uninstall, reinstall doesn?t work, finally I have to use ccleaner for reinstall, I recover the backup copy and now I have the same problem:

Just a word of caution: if it stops working again, remember to delete that line before troubleshooting (or just comment it out) If they change IPs or move you to a different server that fix will fail (and pinging pop.gmail.com will get you the new IP)
I am on74.125.143.108